12.3 ). These myosin projections/cross-bridges can be seen on electron micrographs of skeletal muscle and appear to extend perpendicular from the thick filaments at rest. In the contracted state, the myosin cross-bridges slant toward the center of the sarcomere, which is consistent with the ratchet action of the myosin head.
The cross-bridge cycling mechanism just described is called the sliding filament theory because the myosin
Cycle stops here in relaxed living muscle (because of actions of regulatory mechanisms)
Tension Time0 G. 40 S. 90 msec L.R. 7.5 0 0.5 1.0 •Fig. 12.13. Cross-Bridge Cycle. Intherelaxedstate(statea),ATPispartiallyhydrolyzed(M•ADP•Pi).InthepresenceofelevatedmyoplasmicCa++ (stateb),myosin(M)bindstoactin(A).HydrolysisofATPiscompleted(statec)andcausesaconformationalchangeinthemyosinmoleculethatpullstheactinfilamenttowardthecenterofthesarcomere.AnewATPmoleculebindstomyosinandcausesreleaseofthecross-bridge(stated).PartialhydrolysisofthenewlyboundATPrecocksthemyosinhead,whichisnowreadytobindagainandagain.Ifmyoplasmic[Ca++]isstillelevated,thecyclerepeats.Ifmyoplasmic[Ca++]islow,relaxationresults.
cross-bridge is pulling the actin thin filament toward the center of the sarcomere, which results in an apparent “sliding” of the thin filament past the thick filament. There is, however, uncertainty about how many myosin molecules contribute to the generation of force and whether both myosin heads in a given myosin molecule are involved. It has been calculated that there may be 600 myosin heads per thick filament, with a stoichiometry of 1 myosin head per 1.8 actin molecules. As a result of steric considerations, it is unlikely that all myosin heads can interact with actin, and calculations suggest that even during maximal force generation, only 20% to 40% of the myosin heads bind to actin.
The conversion of chemical energy (i.e., ATP) to mechanical energy by muscle is highly efficient. In isolated muscle preparations, maximum mechanical efficiency (≈65% efficiency) is obtained at a submaximal force of 30% maximal tension. In humans performing steady-state ergometer exercise, mechanical efficiencies range from 40% to 57%.
Skeletal muscle fibers can be classified into two main groups according to the speed of contraction: fast-twitch •Fig. 12.14. A, Musclesvaryintermsofthespeedofcontraction.G,gastrocnemiusmuscleoftheleg;LR,lateralrectusmuscleoftheeye;S,soleusmuscleoftheleg.B, ThespeedofshorteningiscorrelatedwithmyosinATPaseactivity.N-SOL,normalsoleusmuscle(slowtwitch);N-EDL,normalextensordigitorumlongusmuscle(fasttwitch);S-EDL,self-innervatedextensordigitorumlongusmuscle(EDLmotornervetransectedandresutured);S-SOL,self-innervatedsoleusmuscle(soleusmotornervetransectedandresutured);X-EDL,cross-innervatedextensordigitorumlongusmuscle(EDLinnervatedbysoleusmotornerve);X-SOL,cross-innervatedSOLmuscle(soleusinnervatedbyEDLmotornerve).(A, FromMontcastleV[ed].Medical Physiology. 12thed.St.Louis:Mosby;1974.B, FromBárányM,CloseRI.J Physiol. 1971;213:455.) and slow-twitch muscle fibers. As shown in